Alkylation of aromatic compounds produces arylalkanes that may find various commercial uses, e.g., alkylbenzenes that can be sulfonated to produce detergents. In the alkylation process, aromatic compound is reacted with olefin of the desired molecular weight to produce the sought arylalkane. The alkylation conditions comprise a catalyst such as aluminum chloride, hydrogen fluoride, or zeolitic catalysts and elevated temperature.
The alkylation, however, is not selective and can produce dimers, dialkylaryl compounds and diaryl compounds (“heavies”) and can affect skeletal isomerization of the olefin, resulting in a loss of selectivity to the sought arylalkane structure. The formation of dialkylaryl compounds is particularly problematic as the reaction approaches complete conversion of the olefin and the greater concentration of the arylalkane since the likelihood has increased that an olefin molecule will react with an arylalkane molecule rather than a molecule of the aromatic compound in the feed. Accordingly, typical processes use a large excess of aromatic compound to enhance selectivity to arylalkane over dialkylaryl compound. In many instances, the mole ratio of aromatic compound to olefin is greater than 15:1.
In order to provide an economically viable process, the unreacted aromatic compound must be recovered from the alkylation product and recycled. Typical commercial processes recover the aromatic compound during refining the alkylation product through the use of several distillation steps. For instance, see Pujado, Linear Alkylbenzene (LAB) Manufacture, Handbook of Petroleum Refining Processes, Second Edition, pp 1.53 to 1.66 (1996), especially pages 1.56 to 1.60, who discloses refining processes for linear alkylbenzenes. In general, benzene and an olefin-containing feedstock derived from a paraffin dehydrogenation are reacted to produce an alkylation reaction product. The reaction product is refined. A first distillation in a benzene column separates a benzene stream as an overhead stream for recycling to the alkylation reactor. The bottoms stream from the benzene column is virtually free of benzene and is then subjected to a distillation to separate paraffins and unreacted olefin in a paraffins column. The paraffins-containing overhead is capable of being recycled to the paraffin dehydrogenation unit while the bottoms stream is passed to a heavy alkylate distillation column. In the heavy alkylate distillation column, heavies are separated from the lighter alkylbenzene, and a heavies-containing stream is withdrawn as a bottoms stream. If desired, the bottoms stream can be subjected to a further distillation to recover additional alkylbenzene.
An important consideration for commercial-scale facilities for production of arylalkanes, especially alkylbenzene, is energy and equipment integration. For example, reboilers for distillation columns are conventionally heated with a thermal stream, e.g., hot oil or other thermally-stable liquid, derived from a central heater. The capacity of a distillation column, at a given degree of separation, can thus be limited by the availability of thermal fluid. For an alkylbenzene process having a refining system comprising a benzene column, paraffins column and heavy alkylate column, the benzene column consumes the greatest portion of the reboiler heat. Thus the heat demand or reboiler size for the benzene distillation can provide a bottleneck to increased capacity at a given benzene to olefin feed ratio to the alkylation reactor. Similarly, the size of the benzene column itself can pose a bottleneck.
Significant economic benefits can be achieved through even slight improvements in efficiency or reductions in energy consumption or increases in production capacity in a given existing plant, e.g., through debottlenecking, provided that no undue increase in the production of heavies occurs and the arylalkyl meets specifications after refining.
Fritsch, et al., in U.S. Pat. No. 6,069,285 disclose the use of a benzene rectifier and a benzene fractionation column to treat effluent from an aromatic alkylation process using solid alkylation catalyst. The rectifier provides an overhead containing feed aromatics and a rectifier bottom stream comprising feed aromatics and enriched in alkylaromatics. The overhead stream from the rectifier is recycled to an on-stream alkylation reactor. The benzene column produces higher purity benzene-containing overhead stream that can be used to regenerate a sorption bed to treat the olefin-containing feed prior to being passed to the alkylation reactor and can be used to regenerate solid alkylation catalyst in an off-stream alkylation reactor.
Processes and apparatus are sought to effect alkylation that reduce the size and heat demand of the distillation system to remove aromatics from the alkylation reaction product at a given production rate. The processes and apparatus would thus allow the debottlenecking of existing facilities and the design of new facilities with a smaller aromatics removal column.